Melamine triazine ring

Figure 1. Infrared spectra of cured and degraded (230 hours, UV 50) samples of Coating G. Pealcs of interest include A, Acrylic hydroxy B, Melamine methyloi u, 1710 cm Q Melamine-melamine crosslink e,Melamine methoxy Melamine triazine ring G, Styrene.

The chemistry of melamines and phenoHcs is quite similar. In both cases formaldehyde [50-00-0] is added to the reactive sites on the patent ring to form methylol phenols (3) or methylol melamines (4) (see Phenolresins Aminoresins). There ate six reactive sites on the triazine ring of melamine [108-78-1] (1) so it is possible to form hexamethylolmelamine. However, the most common degree of methylolation is 1.5—2.0. The ortho and para positions of phenol ate active thus phenol can be trimethylolated (2). However, as with melamine, lower degrees of methylolation such as 1.2—2.5 ate... 

Melamine resins were introduced about ten years after the Beetle molding compound. They were very similar to those based on urea but had superior quaHties. Henkel in Germany was issued a patent for a melamine resin in 1936 (7). Melamine resins rapidly supplanted urea resins and were soon used in molding, laminating, and bonding formulations, as well as for textile and paper treatments. The remarkable stabiHty of the symmetrical triazine ring made these products resistant to chemical change once the resin had been cured to the insoluble, cross-linked state. 

Manufacture. Dicyandiamide is converted into melamine by heating. Simple pyrolysis above the melting point leads to an exothermic reaction however, deammoniation occurs, forming products containing two or three triazine rings as well as melamine. After it was discovered in 1940 that deammoniation can be counteracted by conducting the reaction under ammonia pressure, various methods were developed to control the exothermic reaction on an industrial scale. 

While there is clear evidence that melam Is the dimer of melamine in which two s-triazine rings are connected by a NH bridge (di 6,[2,4 diamino-1,3,5-triazine]amine) (10) ... 

Melamine is now a very important chemical product since it is a starting substance for the manufacture of plastics. The presence in melamine of a triazine ring, as in cyclonite, and of three amino groups induced attempts to nitrate this substance in order to obtain an explosive nitramine. 

The synthesis of alkoxysilyl-modifted melamine derivatives comprised the formation of the triamino substituted triazine ring from trichloro triazine and simultaneous introduction of a hydrolysable organoalkoxysilyl group. Thus, aminopropyltriethoxysilane or A -methylaminopropyltrimethoxysilane reacts with 2,4,6-trichloro-l,3,5-triazine or with partially A, A -diethylamine-substituted triazine to give the triazine derivatives 7 to 12 (Eq. 4. Table 2). 

The explanation of this inherent flame retardancy is the high char yield generated by the thermal decomposition of organic structures containing triazinic rings. It is well known that melamine is used itself as a flame retardant additive in flexible PU foams. 

Although the reaction mechanism is not yet fully understood, cyanic acid or isocyanic acid seems to be the first decomposition product of urea. Trimerization of cyanic acid leads to cyanuric acid (see section on Cyanuric Acid, p 685). In the presence of catalysts, cyanic acid disproporlionates into carbon dioxide and cyanamide, which then trimerizes to melamine. Thus, decomposition products, and nol urea itself, cyclotrimerize to provide the 1,3,5-triazine rings. These reactions form the basis for the industrial production of cyanuric acid286-301 and melamine.302-315... 

The 1,3,5-triazine ring of melamine (1,3,5-triazine-2,4,6-triamine, l) is very stable and is cleaved only under very vigorous conditions.7,8 When melamine is heated with potassium carbonate or a mixture of sodium and potassium carbonate at 375 450°C in an aluminum vessel, the corresponding alkali cyanates 2 are formed by ring fission.9 A similar conversion is achieved with potassium hydroxide at about 250 °C.8 10 Thermal decomposition of melamine occurs on heating above 600CC,7 and on reaction with sodium amide and carbon at 760-850°C, alkali cyanides are formed.8... 

Acrylated Melamine resins were prepared by reacting different levels of acrylamide with a fully alkylated coetherified melamine resin containing methyl and butyl groups at an approximate ratio of 1 1. They were prepared with 0.5, 1.0, 1.5, and 2.5 moles of acrylamide per triazine ring. Throughout the paper a bis-phenol A epoxy acrylate resin (Ebecryl 3700 from Radcure Specialties) and an aliphatic urethane acrylate resin (Ebecryl 8800 from Radcure Specialties) are used as controls. 

A MUF resin, at parity of all other conditions, yields a lower pH drop after addition of the hardener than a UF resin [46]. This lower drop of the pH due to the buffer capacity of the triazine ring of melamine, however, also eauses a deerease of the hardening rate of the resin and, therefore, a lengthening of its gel time [1], henee a lengthening of the hot press time is necessary. This is also seen in the shifts of the exothermie differential scanning calorimetry (DSC) peak of hardening whieh are observed in thermal experiments [47]. 

The NMR spectrum of the M-DME resin with molar ratio M DME =1 2 shown in Fig. 3 is very similar to that of the U-DME. Major difference is represented by the C=N groups of the triazine ring of the reacted melamine that did not appear in the U-DME spectrum (Table 3). As melamine presents three reactive amine groups and due to the resin molar ratio (M DME = 1 2), a large majority of the DME seemed to have reacted. However, no trace of DME reacted with two... 

Thes-triazine ring is similar to the benzene ring in terms of being highly resonance stabilized, and, therefore, thermally stable. This property was first exploited with the melamine-formaldehyde resins (see also Section 28.3.2), whereby preformed triazine structures are polycondensed. But the triazine ring can also be formed in situ, as, for example, in the cyclotrimerization of nitriles or primary or secondary biscyanamides in ketones or lower alcohols ... 

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